Platelet microparticles are a normal constituent of circulating blood. Several studies have demonstrated positive correlations between thrombotic states and platelet microparticle levels. Yet little is known about the processes by which platelet microparticles are generated in vivo. We now characterize microparticles derived directly from megakaryocytes. Video microscopy of live mouse megakaryocytes demonstrated that microparticles form as submicron beads along the lengths of slender, unbranched micropodia. These microparticles are CD41(+), CD42b(+), and express surface phosphatidylserine. Megakaryocyte microparticle generation is resistant to inhibition of microtubule assembly, which is critical to platelet formation, and augmented by inhibition of actin polymerization. To determine whether circulating microparticles are derived primarily from activated platelets or megakaryocytes, we identified markers that distinguish between these 2 populations. CD62P and LAMP-1 were found only on mouse microparticles from activated platelets. In contrast, full-length filamin A was found in megakaryocyte-derived microparticles, but not microparticles from activated platelets. Circulating microparticles isolated from mice were CD62P(-), LAMP-1(-) and expressed full-length filamin A, indicating a megakaryocytic origin. Similarly, circulating microparticles isolated from healthy volunteers were CD62P(-) and expressed full-length filamin A. Cultured human megakaryocytes elaborated microparticles that were CD41(+), CD42b(+), and express surface phosphatidylserine. These results indicate that direct production by megakaryocytes represents a physiologic means to generate circulating platelet microparticles